1. Field of the Invention
The present invention relates to an image forming method and apparatus for performing a cleaning operation of a transfer member. More specifically, the present invention relates to an electrophotographic image forming apparatus such as a copier, printer, facsimile machine and so forth, in which a transfer voltage applying unit performs a cleaning operation of a transfer member to remove developer from the transfer member, and a method of performing a cleaning operation.
2. Discussion of the Related Art
It is well known that background image forming apparatuses have used a non-contact type corona discharging unit that serves as a transfer unit transferring a toner image formed on an image bearing member onto a transfer medium. For corona discharging, however, the transfer unit needs to be applied with a high voltage, which can produce ozone of high concentration. To avoid the production of ozone of high concentration, background image forming apparatuses employing a transfer method in which a bias voltage is directly applied to a back side of a recording medium have been increasing in recent years. The above-described contact type transfer method uses less voltage than that used for a corona discharging unit, and therefore, produces a less amount of ozone. Further, since a recording medium receives a toner image while being held in contact between a contact type transfer unit and an image bearing member, the recording medium is closely attached to the image bearing member with high adherence.
In the above-described background image forming apparatus employing the contact type transfer method, however, the contact type transfer unit directly contacts the image bearing member, and therefore, toner used for forming a toner image on the image bearing member may adhere to the transfer unit. For example, when a paperjam occurs at an upstream portion in a sheet conveyance direction of a transfer nip portion where the image is transferred onto the recording medium, a toner image formed on the image bearing member can reach the transfer nip portion but the recording medium cannot due to the paper jam. On the other hand, the toner used for the toner image formed on the image bearing member may be conveyed to the transfer nip portion and may be directly transferred onto the contact type transfer unit. In this case, it is possible that the toner directly transferred onto the contact type transfer unit can contaminate the back side of a next recording medium and/or can change a value of a resistance voltage of the contact transfer unit, which may cause a partial transfer failure.
Specifically, when a full color printer forms a plurality of toner layers on a transfer member, a large amount of toner adhered to a contact type transfer unit may cause a great adverse effect to toner contamination on the back side of a recording medium.
To prevent the problem described above, some techniques have been proposed.
In one technique, a cleaning unit performs a cleaning operation by contacting a cleaning blade and/or a fur brush included therein with a contact type transfer unit. The above-described cleaning unit that is held in contact with the transfer unit can constantly perform a cleaning operation with respect to the contact type transfer unit, therefore, there is no need to include a sequence such as a cleaning mode. However, the above-described contact type cleaning method using the cleaning blade and fur brush to be held in contact with the transfer unit may cause following problems.
Since the cleaning blade is constantly held in contact with the contact type transfer unit, an edge portion of the cleaning blade can easily wear. For maintaining the cleaning ability of the cleaning unit, the cleaning blade by itself or with its peripheral members may need to be replaced constantly. Further, if the friction coefficient of the surface of the contact type transfer unit increases, the cleaning blade may be curled.
To resolve the above-described problem, another image forming apparatus has employed a technique wherein a lubricant is applied to coat a portion outside an image forming area. The technique works well to prevent curling of an edge portion of a cleaning blade, but cannot help prevent wear of the cleaning blade contacting the image forming area.
Another image forming apparatus, which does not use a lubricant, employs a technique wherein a toner image is formed on a non-image forming area to prevent curling of a cleaning blade so as to utilize the oil content in the toner serving as a lubricant. However, the technique contributes to an increase of toner consumption to waste unused toner, which may not be economical.
There is another image forming apparatus employing a technique that does not employ the above-described contact type cleaning method. When a recording medium is not positioned on a transfer portion, a transfer unit rotates at least one cycle, and at the same time, a voltage having a predetermined polarity is applied to the transfer unit. Further, when the transfer unit rotates at least another cycle, a voltage having a polarity opposite to the predetermined polarity is applied to the transfer unit.
In another image forming apparatus using a technique wherein when a cleaning operation of a transfer roller is performed, the polarity of a cleaning bias is switched between positive charge and negative charge.
Further in another image forming apparatus using a technique wherein the polarity of a cleaning bias to be applied to a transfer roller can be switched while a cleaning unit and a developing unit are started and before a transfer medium reaches a transfer portion at the start of an image forming operation.
However, in the above-described techniques, the polarity of a bias voltage is switched, and then the transfer roller is rotated one or more cycles to be applied with a bias voltage at each job. The above-described operation requires a great amount of period of time in total to be applied with a bias voltage, which can contribute to deterioration of productivity.